Tone: serious, anee-san, deep.
An example of what we are looking for would be Ishizu's (YuGiOh) serious voice:
https://www.youtube.com/watch?v=xOJr1dTPSis

Komachi High’s Ice Queen and dutiful Priestess. As the town of Komachi was built upon traditions, Emiri became one of their figureheads the moment she inherited her family’s shrine. Quiet and serious, Emiri always puts her responsibility above everything and everyone else.